Similarities are established between well‐known reactivity descriptors of metal electrodes for their activity in the oxygen reduction reaction (ORR) and the reactivity of molecular catalysts, in ...particular macrocyclic MN4 metal complexes confined to electrode surfaces. We show that there is a correlation between the MIII/MII redox potential of MN4 chelates and the M‐O2 binding energies. Specifically, the binding energy of O2 (and other O species) follows the MIII‐OH/MII redox transition for MnN4 and FeN4 chelates. The ORR volcano plot for MN4 catalysts is similar to that for metal catalysts: catalysts on the weak binding side (mostly CoN4 chelates) yield mainly H2O2 as the product, with an ORR onset potential independent of the pH value on the NHE scale (and therefore pH‐dependent on the RHE scale); catalysts on the stronger binding side yield H2O as the product with the expected pH‐dependence on the NHE scale. The suggested descriptors also apply to heat‐treated pyrolyzed MN4 catalysts.
Descriptive knowledge: Reactivity descriptors for molecular MN4 catalysts of the oxygen reduction reaction are described, including donor–acceptor hardness, M‐O2 binding energies, and the M+n/M+(n−1) formal potentials of the catalysts. It is shown that the last two descriptors are directly correlated to each other. The mechanisms for oxygen reduction on MN4 resemble those on metal catalysts in terms of both pH dependence and product formation.
This paper summarizes the thermodynamic theory of multi-electron transfer reactions and its implications for electrocatalysis. We discuss the fundamental differences between catalyzing reactions ...involving the transfer of one electron or no catalytic intermediates, two electron transfers with one catalytic intermediate, two electron transfer with two catalytic intermediates, and more than two electron transfers with more than one intermediate. These different classes of reactions imply different optimization problems for finding the best catalyst, dictated primarily by the thermodynamics of binding of the catalytic intermediates. The application of this theory to hydrogen evolution and oxidation, oxygen evolution and reduction, and carbon dioxide reduction, is discussed.
Selectivity between chlorine evolution and oxygen evolution in aqueous media is a phenomenon of central importance in the chlor-alkali process, water treatment, and saline water splitting, which is ...an emerging technology for sustainable energy conversion. An apparent scaling between oxygen vs. chlorine evolution has been established, making it challenging to carry the two reactions out individually with 100% faradaic efficiency. To aid selectivity determination, we developed a new method to quickly measure chlorine evolution rates using a conventional RRDE setup. We showed that a Pt ring fixed at 0.95V vs. RHE in pH0.88 can selectively reduce the Cl2 formed on the disk and this allows precise and flexible data acquisition. Using this method, we demonstrate that oxygen evolution and chlorine evolution on a glassy carbon supported IrOx catalyst proceed independently, and that the selectivity towards chlorine evolution (εCER) rapidly approaches 100% as Cl− increases from 0 to 100mM. Our results suggest that on IrOx, oxygen evolution is not suppressed or influenced by the presence of Cl− or by the chlorine evolution reaction taking place simultaneously on the surface.
Fueling the future: A fibrillar network (red fibers, see figure) is formed from an activated building block (red), which is obtained from a synthetic gelator (blue) in a dissipative self‐assembly ...process that is fueled by an alkylating agent. When the available energy is depleted, the system reverts to its thermodynamic equilibrium, that is, an isotropic solution.
The electrocatalytic oxidation of formic acid (HCOOH) and formate (HCOO−) to CO2 on platinum has been studied over a wide range of pH (0–12) by surface-enhanced infrared absorption spectroscopy ...(SEIRAS) coupled with cyclic voltammetry. The peak current of HCOOH/HCOO− oxidation exhibits a volcano-shaped pH dependence peaked at a pH close to the pKa of HCOOH (3.75). The experimental result is reasonably explained by a simple kinetic model that HCOO− oxidation is the dominant reaction route over the whole pH range. HCOOH is oxidized after being converted to HCOO− via the acid-base equilibrium. The ascending part of the volcano plot at pH<4 is ascribed mostly to the increase of the molar ratio of HCOO−, while the descending part at pH>4 is ascribed to the suppression of HCOO− oxidation by adsorbed OH or oxidation of the electrode surface. In acidic media, HCOOH is adsorbed on the electrode as formate with a bridge-bonded configuration. The bridge-bonded adsorbed formate is stable and suppresses HCOO− oxidation by blocking active site. However, the suppression is not fatal because bridge-bonded adsorbed formate enhances the oxidation of HCOO− at high potential by suppressing the adsorption of OH or surface oxidation. The complex cyclic voltammograms for HCOOH/HCOO− oxidation also can be well interpreted in terms of the simple kinetic model. The experimental results presented here serve as a generic example illustrating the importance of pH variations in catalytic proton-coupled electron transfer reactions.
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